src/compiler/js-context-specialization.cc

// Copyright 2014 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include "src/compiler/js-context-specialization.h"

#include "src/base/logging.h"
#include "src/compiler/access-builder.h"
#include "src/compiler/common-operator.h"
#include "src/compiler/compilation-dependencies.h"
#include "src/compiler/feedback-source.h"
#include "src/compiler/js-graph.h"
#include "src/compiler/js-heap-broker.h"
#include "src/compiler/js-operator.h"
#include "src/compiler/linkage.h"
#include "src/compiler/node-properties.h"
#include "src/compiler/property-access-builder.h"
#include "src/compiler/simplified-operator.h"
#include "src/deoptimizer/deoptimize-reason.h"
#include "src/objects/contexts-inl.h"
#include "src/objects/contexts.h"
#include "src/objects/property-cell.h"

namespace v8 {
namespace internal {
namespace compiler {

Reduction JSContextSpecialization::Reduce(Node* node) {
  switch (node->opcode()) {
    case IrOpcode::kParameter:
      return ReduceParameter(node);
    case IrOpcode::kJSLoadContextNoCell:
      return ReduceJSLoadContextNoCell(node);
    case IrOpcode::kJSLoadContext:
      return ReduceJSLoadContext(node);
    case IrOpcode::kJSStoreContextNoCell:
      return ReduceJSStoreContextNoCell(node);
    case IrOpcode::kJSStoreContext:
      return ReduceJSStoreContext(node);
    case IrOpcode::kJSGetImportMeta:
      return ReduceJSGetImportMeta(node);
    default:
      break;
  }
  return NoChange();
}

Reduction JSContextSpecialization::ReduceParameter(Node* node) {
  DCHECK_EQ(IrOpcode::kParameter, node->opcode());
  int const index = ParameterIndexOf(node->op());
  if (index == Linkage::kJSCallClosureParamIndex) {
    // Constant-fold the function parameter {node}.
    Handle<JSFunction> function;
    if (closure().ToHandle(&function)) {
      Node* value =
          jsgraph()->ConstantNoHole(MakeRef(broker_, function), broker());
      return Replace(value);
    }
  }
  return NoChange();
}

Reduction JSContextSpecialization::SimplifyJSLoadContextNoCell(
    Node* node, Node* new_context, size_t new_depth) {
  DCHECK_EQ(IrOpcode::kJSLoadContextNoCell, node->opcode());
  const ContextAccess& access = ContextAccessOf(node->op());
  DCHECK_LE(new_depth, access.depth());

  if (new_depth == access.depth() &&
      new_context == NodeProperties::GetContextInput(node)) {
    return NoChange();
  }

  const Operator* op = jsgraph_->javascript()->LoadContextNoCell(
      new_depth, access.index(), access.immutable());
  NodeProperties::ReplaceContextInput(node, new_context);
  NodeProperties::ChangeOp(node, op);
  return Changed(node);
}

Reduction JSContextSpecialization::SimplifyJSLoadContext(Node* node,
                                                         Node* new_context,
                                                         size_t new_depth) {
  DCHECK_EQ(IrOpcode::kJSLoadContext, node->opcode());
  const ContextAccess& access = ContextAccessOf(node->op());
  DCHECK_LE(new_depth, access.depth());

  if (new_depth == access.depth() &&
      new_context == NodeProperties::GetContextInput(node)) {
    return NoChange();
  }

  const Operator* op =
      jsgraph_->javascript()->LoadContext(new_depth, access.index());
  NodeProperties::ReplaceContextInput(node, new_context);
  NodeProperties::ChangeOp(node, op);
  return Changed(node);
}

Reduction JSContextSpecialization::SimplifyJSStoreContextNoCell(
    Node* node, Node* new_context, size_t new_depth) {
  DCHECK_EQ(IrOpcode::kJSStoreContextNoCell, node->opcode());
  const ContextAccess& access = ContextAccessOf(node->op());
  DCHECK_LE(new_depth, access.depth());

  if (new_depth == access.depth() &&
      new_context == NodeProperties::GetContextInput(node)) {
    return NoChange();
  }

  const Operator* op =
      jsgraph_->javascript()->StoreContextNoCell(new_depth, access.index());
  NodeProperties::ReplaceContextInput(node, new_context);
  NodeProperties::ChangeOp(node, op);
  return Changed(node);
}

Reduction JSContextSpecialization::SimplifyJSStoreContext(Node* node,
                                                          Node* new_context,
                                                          size_t new_depth) {
  DCHECK_EQ(IrOpcode::kJSStoreContext, node->opcode());
  const ContextAccess& access = ContextAccessOf(node->op());
  DCHECK_LE(new_depth, access.depth());

  if (new_depth == access.depth() &&
      new_context == NodeProperties::GetContextInput(node)) {
    return NoChange();
  }

  const Operator* op =
      jsgraph_->javascript()->StoreContext(new_depth, access.index());
  NodeProperties::ReplaceContextInput(node, new_context);
  NodeProperties::ChangeOp(node, op);
  return Changed(node);
}

namespace {

bool IsContextParameter(Node* node) {
  DCHECK_EQ(IrOpcode::kParameter, node->opcode());
  return ParameterIndexOf(node->op()) ==
         StartNode{NodeProperties::GetValueInput(node, 0)}
             .ContextParameterIndex_MaybeNonStandardLayout();
}

// Given a context {node} and the {distance} from that context to the target
// context (which we want to read from or store to), try to return a
// specialization context.  If successful, update {distance} to whatever
// distance remains from the specialization context.
OptionalContextRef GetSpecializationContext(JSHeapBroker* broker, Node* node,
                                            size_t* distance,
                                            Maybe<OuterContext> maybe_outer) {
  switch (node->opcode()) {
    case IrOpcode::kHeapConstant: {
      // TODO(jgruber,chromium:1209798): Using kAssumeMemoryFence works around
      // the fact that the graph stores handles (and not refs). The assumption
      // is that any handle inserted into the graph is safe to read; but we
      // don't preserve the reason why it is safe to read. Thus we must
      // over-approximate here and assume the existence of a memory fence. In
      // the future, we should consider having the graph store ObjectRefs or
      // ObjectData pointer instead, which would make new ref construction here
      // unnecessary.
      HeapObjectRef object =
          MakeRefAssumeMemoryFence(broker, HeapConstantOf(node->op()));
      if (object.IsContext()) return object.AsContext();
      break;
    }
    case IrOpcode::kParameter: {
      OuterContext outer;
      if (maybe_outer.To(&outer) && IsContextParameter(node) &&
          *distance >= outer.distance) {
        *distance -= outer.distance;
        return MakeRef(broker, outer.context);
      }
      break;
    }
    default:
      break;
  }
  return OptionalContextRef();
}

}  // anonymous namespace

Reduction JSContextSpecialization::ReduceJSLoadContextNoCell(Node* node) {
  DCHECK_EQ(IrOpcode::kJSLoadContextNoCell, node->opcode());

  const ContextAccess& access = ContextAccessOf(node->op());
  size_t depth = access.depth();

  // First walk up the context chain in the graph as far as possible.
  Node* context = NodeProperties::GetOuterContext(node, &depth);

  OptionalContextRef maybe_concrete =
      GetSpecializationContext(broker(), context, &depth, outer());
  if (!maybe_concrete.has_value()) {
    // We do not have a concrete context object, so we can only partially reduce
    // the load by folding-in the outer context node.
    return SimplifyJSLoadContextNoCell(node, context, depth);
  }

  // Now walk up the concrete context chain for the remaining depth.
  ContextRef concrete = maybe_concrete.value();
  concrete = concrete.previous(broker(), &depth);
  if (depth > 0) {
    TRACE_BROKER_MISSING(broker(), "previous value for context " << concrete);
    return SimplifyJSLoadContextNoCell(
        node, jsgraph()->ConstantNoHole(concrete, broker()), depth);
  }

  if (!access.immutable() &&
      !broker()->dependencies()->DependOnContextCell(
          concrete, access.index(), ContextCell::kConst, broker())) {
    // We found the requested context object but since the context slot is
    // mutable we can only partially reduce the load.
    return SimplifyJSLoadContextNoCell(
        node, jsgraph()->ConstantNoHole(concrete, broker()), depth);
  }

  // This will hold the final value, if we can figure it out.
  OptionalObjectRef maybe_value;
  maybe_value = concrete.get(broker(), static_cast<int>(access.index()));

  if (!maybe_value.has_value()) {
    TRACE_BROKER_MISSING(broker(), "slot value " << access.index()
                                                 << " for context "
                                                 << concrete);
    return SimplifyJSLoadContextNoCell(
        node, jsgraph()->ConstantNoHole(concrete, broker()), depth);
  }

  // Even though the context slot is immutable, the context might have escaped
  // before the function to which it belongs has initialized the slot.
  // We must be conservative and check if the value in the slot is currently
  // the hole or undefined. Only if it is neither of these, can we be sure
  // that it won't change anymore.
  if (maybe_value->IsUndefined() || maybe_value->IsTheHole()) {
    return SimplifyJSLoadContextNoCell(
        node, jsgraph()->ConstantNoHole(concrete, broker()), depth);
  }

  // Success. The context load can be replaced with the constant.
  Node* constant = jsgraph_->ConstantNoHole(*maybe_value, broker());
  ReplaceWithValue(node, constant);
  return Replace(constant);
}

Reduction JSContextSpecialization::ReduceJSLoadContext(Node* node) {
  DCHECK_EQ(IrOpcode::kJSLoadContext, node->opcode());

  const ContextAccess& access = ContextAccessOf(node->op());
  DCHECK(!access.immutable());
  size_t depth = access.depth();

  // First walk up the context chain in the graph as far as possible.
  Node* effect = NodeProperties::GetEffectInput(node);
  Node* control = NodeProperties::GetControlInput(node);
  Node* context = NodeProperties::GetOuterContext(node, &depth);

  OptionalContextRef maybe_concrete =
      GetSpecializationContext(broker(), context, &depth, outer());
  if (!maybe_concrete.has_value()) {
    // We do not have a concrete context object, so we can only partially reduce
    // the load by folding-in the outer context node.
    return SimplifyJSLoadContext(node, context, depth);
  }

  // Now walk up the concrete context chain for the remaining depth.
  ContextRef concrete = maybe_concrete.value();
  concrete = concrete.previous(broker(), &depth);
  if (depth > 0) {
    TRACE_BROKER_MISSING(broker(), "previous value for context " << concrete);
    return SimplifyJSLoadContext(
        node, jsgraph()->ConstantNoHole(concrete, broker()), depth);
  }

  auto maybe_value = concrete.get(broker(), static_cast<int>(access.index()));
  if (!maybe_value || maybe_value->IsTheHole() ||
      maybe_value->IsUndefinedContextCell()) {
    return SimplifyJSLoadContext(
        node, jsgraph()->ConstantNoHole(concrete, broker()), depth);
  }
  if (!maybe_value->IsContextCell()) {
    // Do a normal context load.
    Node* load = effect = jsgraph_->graph()->NewNode(
        jsgraph_->simplified()->LoadField(
            AccessBuilder::ForContextSlot(access.index())),
        jsgraph_->ConstantNoHole(concrete, broker()), effect, control);
    ReplaceWithValue(node, load, effect, control);
    return Changed(node);
  }
  DCHECK(concrete.object()->HasContextCells());

  compiler::ContextCellRef slot_ref = maybe_value->AsContextCell();
  // TODO(victorgomes): Make DependOnContextCell return the state instead.
  ContextCell::State state = slot_ref.state();
  switch (state) {
    case ContextCell::kConst: {
      auto constant = slot_ref.tagged_value(broker());
      if (!constant.has_value()) {
        return SimplifyJSLoadContext(
            node, jsgraph()->ConstantNoHole(concrete, broker()), depth);
      }
      broker()->dependencies()->DependOnContextCell(slot_ref, state);
      ReplaceWithValue(node, jsgraph_->ConstantNoHole(*constant, broker()),
                       effect, control);
      return Changed(node);
    }
    case ContextCell::kSmi: {
      broker()->dependencies()->DependOnContextCell(slot_ref, state);
      Node* load = effect = jsgraph_->graph()->NewNode(
          jsgraph_->simplified()->LoadField(
              AccessBuilder::ForContextCellTaggedValue()),
          jsgraph_->ConstantNoHole(slot_ref, broker()), effect, control);
      ReplaceWithValue(node, load, effect, control);
      return Changed(node);
    }
    case ContextCell::kInt32: {
      broker()->dependencies()->DependOnContextCell(slot_ref, state);
      Node* int32_load = effect = jsgraph_->graph()->NewNode(
          jsgraph_->simplified()->LoadField(
              AccessBuilder::ForContextCellInt32Value()),
          jsgraph_->ConstantNoHole(slot_ref, broker()), effect, control);
      ReplaceWithValue(node, int32_load, effect, control);
      return Changed(node);
    }
    case ContextCell::kFloat64: {
      broker()->dependencies()->DependOnContextCell(slot_ref, state);
      Node* double_load = effect = jsgraph_->graph()->NewNode(
          jsgraph_->simplified()->LoadField(
              AccessBuilder::ForContextCellFloat64Value()),
          jsgraph_->ConstantNoHole(slot_ref, broker()), effect, control);
      ReplaceWithValue(node, double_load, effect, control);
      return Changed(node);
    }
    case ContextCell::kDetached: {
      Node* load = effect = jsgraph_->graph()->NewNode(
          jsgraph_->simplified()->LoadField(
              AccessBuilder::ForContextSlot(access.index())),
          jsgraph_->ConstantNoHole(concrete, broker()), effect, control);
      ReplaceWithValue(node, load, effect, control);
      return Changed(node);
    }
  }
  UNREACHABLE();
}

Reduction JSContextSpecialization::ReduceJSStoreContextNoCell(Node* node) {
  DCHECK_EQ(IrOpcode::kJSStoreContextNoCell, node->opcode());

  const ContextAccess& access = ContextAccessOf(node->op());
  size_t depth = access.depth();

  // First walk up the context chain in the graph until we reduce the depth to 0
  // or hit a node that does not have a CreateXYZContext operator.
  Node* context = NodeProperties::GetOuterContext(node, &depth);

  OptionalContextRef maybe_concrete =
      GetSpecializationContext(broker(), context, &depth, outer());
  if (!maybe_concrete.has_value()) {
    // We do not have a concrete context object, so we can only partially reduce
    // the load by folding-in the outer context node.
    return SimplifyJSStoreContextNoCell(node, context, depth);
  }

  // Now walk up the concrete context chain for the remaining depth.
  ContextRef concrete = maybe_concrete.value();
  concrete = concrete.previous(broker(), &depth);
  if (depth > 0) {
    TRACE_BROKER_MISSING(broker(), "previous value for context " << concrete);
    return SimplifyJSStoreContextNoCell(
        node, jsgraph()->ConstantNoHole(concrete, broker()), depth);
  }

  return SimplifyJSStoreContextNoCell(
      node, jsgraph()->ConstantNoHole(concrete, broker()), depth);
}

Reduction JSContextSpecialization::ReduceJSStoreContext(Node* node) {
  DCHECK(v8_flags.script_context_cells || v8_flags.function_context_cells);
  DCHECK_EQ(IrOpcode::kJSStoreContext, node->opcode());

  const ContextAccess& access = ContextAccessOf(node->op());
  size_t depth = access.depth();

  // First walk up the context chain in the graph until we reduce the depth to 0
  // or hit a node that does not have a CreateXYZContext operator.
  Node* context = NodeProperties::GetOuterContext(node, &depth);
  Node* value = NodeProperties::GetValueInput(node, 0);
  Effect effect{NodeProperties::GetEffectInput(node)};
  Control control{NodeProperties::GetControlInput(node)};

  OptionalContextRef maybe_concrete =
      GetSpecializationContext(broker(), context, &depth, outer());
  if (!maybe_concrete.has_value()) {
    // We do not have a concrete context object, so we can only partially reduce
    // the load by folding-in the outer context node.
    return SimplifyJSStoreContext(node, context, depth);
  }

  // Now walk up the concrete context chain for the remaining depth.
  ContextRef concrete = maybe_concrete.value();
  concrete = concrete.previous(broker(), &depth);
  if (depth > 0) {
    TRACE_BROKER_MISSING(broker(), "previous value for context " << concrete);
    return SimplifyJSStoreContext(
        node, jsgraph()->ConstantNoHole(concrete, broker()), depth);
  }

  auto maybe_value = concrete.get(broker(), static_cast<int>(access.index()));
  if (!maybe_value || maybe_value->IsTheHole() ||
      maybe_value->IsUndefinedContextCell()) {
    return SimplifyJSStoreContext(
        node, jsgraph()->ConstantNoHole(concrete, broker()), depth);
  }
  if (!maybe_value->IsContextCell()) {
    // Do a normal context store.
    Node* store = jsgraph()->graph()->NewNode(
        jsgraph()->simplified()->StoreField(
            AccessBuilder::ForContextSlot(access.index())),
        jsgraph()->ConstantNoHole(concrete, broker()), value, effect, control);
    ReplaceWithValue(node, store, store, control);
    return Changed(node);
  }
  DCHECK(concrete.object()->HasContextCells());

  compiler::ContextCellRef slot_ref = maybe_value->AsContextCell();
  PropertyAccessBuilder access_builder(jsgraph(), broker());

  ContextCell::State state = slot_ref.state();
  switch (state) {
    case ContextCell::kConst: {
      auto constant = slot_ref.tagged_value(broker());
      if (!constant.has_value() ||
          (constant->IsString() && !constant->IsInternalizedString())) {
        return SimplifyJSStoreContext(
            node, jsgraph()->ConstantNoHole(concrete, broker()), depth);
      }
      broker()->dependencies()->DependOnContextCell(slot_ref, state);
      access_builder.BuildCheckValue(value, &effect, control, *constant);
      ReplaceWithValue(node, effect, effect, control);
      return Changed(node);
    }
    case ContextCell::kSmi: {
      broker()->dependencies()->DependOnContextCell(slot_ref, state);
      Node* smi_value = access_builder.BuildCheckSmi(value, &effect, control);
      Node* smi_store = jsgraph()->graph()->NewNode(
          jsgraph()->simplified()->StoreField(
              AccessBuilder::ForContextCellTaggedValue()),
          jsgraph()->ConstantNoHole(slot_ref, broker()), smi_value, effect,
          control);
      ReplaceWithValue(node, smi_store, smi_store, control);
      return Changed(node);
    }
    case ContextCell::kInt32: {
      broker()->dependencies()->DependOnContextCell(slot_ref, state);
      Node* input_number =
          access_builder.BuildCheckNumberFitsInt32(value, &effect, control);
      Node* double_store = jsgraph()->graph()->NewNode(
          jsgraph()->simplified()->StoreField(
              AccessBuilder::ForContextCellInt32Value()),
          jsgraph_->ConstantNoHole(slot_ref, broker()), input_number, effect,
          control);
      ReplaceWithValue(node, double_store, double_store, control);
      return Changed(node);
    }
    case ContextCell::kFloat64: {
      broker()->dependencies()->DependOnContextCell(slot_ref, state);
      Node* input_number =
          access_builder.BuildCheckNumber(value, &effect, control);
      Node* double_store = jsgraph()->graph()->NewNode(
          jsgraph()->simplified()->StoreField(
              AccessBuilder::ForContextCellFloat64Value()),
          jsgraph_->ConstantNoHole(slot_ref, broker()), input_number, effect,
          control);
      ReplaceWithValue(node, double_store, double_store, control);
      return Changed(node);
    }
    case ContextCell::kDetached: {
      Node* store = jsgraph()->graph()->NewNode(
          jsgraph()->simplified()->StoreField(
              AccessBuilder::ForContextSlot(access.index())),
          jsgraph()->ConstantNoHole(concrete, broker()), value, effect,
          control);
      ReplaceWithValue(node, store, store, control);
      return Changed(node);
    }
  }
  UNREACHABLE();
}

OptionalContextRef GetModuleContext(JSHeapBroker* broker, Node* node,
                                    Maybe<OuterContext> maybe_context) {
  size_t depth = std::numeric_limits<size_t>::max();
  Node* context = NodeProperties::GetOuterContext(node, &depth);

  auto find_context = [broker](ContextRef c) {
    while (c.map(broker).instance_type() != MODULE_CONTEXT_TYPE) {
      size_t depth = 1;
      c = c.previous(broker, &depth);
      CHECK_EQ(depth, 0);
    }
    return c;
  };

  switch (context->opcode()) {
    case IrOpcode::kHeapConstant: {
      // TODO(jgruber,chromium:1209798): Using kAssumeMemoryFence works around
      // the fact that the graph stores handles (and not refs). The assumption
      // is that any handle inserted into the graph is safe to read; but we
      // don't preserve the reason why it is safe to read. Thus we must
      // over-approximate here and assume the existence of a memory fence. In
      // the future, we should consider having the graph store ObjectRefs or
      // ObjectData pointer instead, which would make new ref construction here
      // unnecessary.
      HeapObjectRef object =
          MakeRefAssumeMemoryFence(broker, HeapConstantOf(context->op()));
      if (object.IsContext()) {
        return find_context(object.AsContext());
      }
      break;
    }
    case IrOpcode::kParameter: {
      OuterContext outer;
      if (maybe_context.To(&outer) && IsContextParameter(context)) {
        return find_context(MakeRef(broker, outer.context));
      }
      break;
    }
    default:
      break;
  }

  return OptionalContextRef();
}

Reduction JSContextSpecialization::ReduceJSGetImportMeta(Node* node) {
  OptionalContextRef maybe_context = GetModuleContext(broker(), node, outer());
  if (!maybe_context.has_value()) return NoChange();

  ContextRef context = maybe_context.value();
  OptionalObjectRef module = context.get(broker(), Context::EXTENSION_INDEX);
  if (!module.has_value()) return NoChange();
  OptionalObjectRef import_meta =
      module->AsSourceTextModule().import_meta(broker());
  if (!import_meta.has_value()) return NoChange();
  if (import_meta->IsTheHole()) {
    // The import.meta object has not yet been created. Let JSGenericLowering
    // replace the operator with a runtime call.
    return NoChange();
  }
  DCHECK(import_meta->IsJSObject());

  Node* import_meta_const = jsgraph()->ConstantNoHole(*import_meta, broker());
  ReplaceWithValue(node, import_meta_const);
  return Changed(import_meta_const);
}

Isolate* JSContextSpecialization::isolate() const {
  return jsgraph()->isolate();
}

}  // namespace compiler
}  // namespace internal
}  // namespace v8